12 HANDBOOK OF ELECTRICAL ENGINEERING
IfFois designed to be less than approximately 105% then the generators will be able to
absorb the overload until some corrective action by an operator is taken e.g. puts the spare generator
into service.
However, it is also possible to introduce a high-speed load shedding scheme into the power
system whenFois found to be above 105%. Such a scheme will compute in an anticipatory manner
how much consumption should be deleted in the event of a loss of one generator. The designer will
be able to predetermine enough low priority consumers to achieve the necessary corrective action.
See Chapter 16.
The application of theN−1 philosophy is less complicated with incoming feeders e.g. under-
ground cables, overhead lines.Nis usually chosen as 2 because it is not usually economical to use
three or more feeders for one switchboard. Both feeders are usually in service and so the ‘spare’
does not usually exist. However, each feeder is rated to carry the full demand of the switchboard.
Therefore with both in service each one carries half of the demand, and can rapidly take the full
demand if one is switched out of service. This approach also enables a feeder to be taken out service
for periodic maintenance, without disturbing the consumers.
1.4 Standby Capacity of Plain Cable Feeders and Transformer Feeders
In sub-section 1.2 the three ways of considering consumers were discussed, and the terms, vital,
essential and non-essential were introduced. Because of the sensitive nature of the vital and essential
consumers with regard to personnel safety and production continuity, it is established practice to
supply their associated switchboards with dual, or occasionally triple, feeders. For non-essential
switchboards it may be practical to use only one feeder.
For switchboards other than those for the generator or intake feeders it is established practice to
add some margin in power capacity of their feeders so that some future growth can be accommodated.
The margin is often chosen to be 25% above the TPPL.
If the feeders are plain cables or overhead lines then it is a simple matter to choose their
cross-sectional areas to match the current at the 125% duty.
For transformer feeders there are two choices that are normally available. Most power trans-
formers can be fitted with external cooling fans, provided the attachments for these fans are included
in the original purchase order. It is common practice to order transformers initially without fans
and operate them as ONAN until the demand increases to justify the fan cooling. Thereafter the
transformer is operated as ONAF, see sub-section 6.5. Adding fans can increase the capacity of
the transformer by 25% to 35%, depending upon the particular design and ambient conditions. The
alternative choice is simply to rate the ONAN transformer for the 125% duty, and initially oper-
ate it at a lower level. The decision is often a matter of economics and an uncertainty about the
future growth.
When standby or future capacity is required for transformers it is necessary to rate the sec-
ondary cables or busbars correctly at the design stage of the project. Likewise the secondary circuit
breakers and switchgear busbars need to be appropriately rated for the future demand. The decision
to over-rate the primary cables or lines may be made at the beginning of the project or later when
demand increases. Again this is a matter of economics and forecasting demand.